Optical amplifier with wide flat gain dynamic range

ABSTRACT

An optical amplifier comprises an optical fiber segment doped with impurity ions such as erbium for providing optical gain for an optical signal propagating in the optical fiber segment. A first source of a first pumping wavelength pumps the ions from a first ground state to a second metastable state. The metastable state decays to the ground state by stimulated emission to provide the optical gain. A second source of a second pumping wavelength pumps the ions from the ground state to a third auxiliary state. The auxiliary state decays to the metastable state. Thus, by controlling the pumping power in one or both pumping wavelengths, it is possible to control the fraction of ions in the metastable state. This in turn permits control of an overall scale factor of the gain spectrum without substantially affecting the shape of the gain spectrum.

This application claims the benefit of provisional application Ser. No.60/080,128, filed Mar. 31, 1998.

FIELD OF THE INVENTION

The present invention relates to an optical fiber amplifier and, inparticular, to a method for varying an overall scale factor of the gainspectrum of the optical fiber amplifier without substantially changingthe shape of the gain spectrum. Thus, while in a conventional erbiumdoped fiber amplifier (EDFA), the magnitude and shape of the gainspectrum are completely coupled, the present invention provides atechnique for decoupling the magnitude and shape of the gain spectrum ofan EDFA.

BACKGROUND OF THE INVENTION

Achieving a uniform or flat gain spectrum is desirable for opticalamplifiers used in wavelength-multiplexed communication systems. Thegain flatness of an EDFA can be optimized by using host glass materialsfor the erbium that produce flat gain spectra (e.g., aluminum codopedsilica or fluoride glasses such as ZBLAN) and operating the amplifier atan average inversion that provides optimally flat gain in the spectralregion of interest (C. R. Giles and D. J. D. Giovanni, “Spectraldependence of gain and noise in erbium-doped fiber amplifiers,” IEEEPhotonics Technology Letters, vol. 2, pp. 797-800, 1990). The gainflatness can further be improved through the use of gain flatteningfilters (M. Tachibana, R. I. Laming, P. R. Morkel, and D. N. Payne,“Erbium-doped fiber amplifier with flattened gain spectrum,” IEEEPhotonics Technology Letters, vol. 3, pp. 118-120, 1991). All of thesetechniques, however, only provide the optimum gain flatness at a singlegain value (i.e., gain at any particular wavelength). It is well knownthat if the gain of an EDFA is changed by changing the inversion (e.g.,by changing the relative pumping rate), the gain changes in a welldefined spectrally dependent manner (C. R. Giles and D. J. D. Giovanni,“Spectral dependence of gain and noise in erbium-doped fiberamplifiers,” IEEE Photonics Technology Letters, vol. 2, pp. 797-800,1990; J. Nilsson, Y. W. Lee, and W. H. Choe, “Erbium doped fibreamplifier with dynamic gain flatness for WDM.,” Electronics Letters,vol. 31, pp. 1578-1579, 1995). As a result, if a conventional EDFA isused in an application where its gain needs to be different from theflattest gain of the amplifier, its gain spectrum will show excessnormalized gain ripple ((maximum gain—minimum gain)/minimum gain ascalculated on the wavelength band of interest).

An example of how this can be a problem is provided by an opticallyamplified fiber transmission system where one needs to support fiberspans shorter than those for which the amplifier is designed. It istypically impractical to have separate amplifiers custom designed foreach fiber span. Therefore, one is either forced to have an amplifierwith a distorted gain spectrum or to add enough loss to the system thatthe design gain is actually needed from the amplifier. The latter usesmore optical power than a redesigned amplifier would and has inferiornoise performance, even if the loss is added between stages of an EDFA(Y. Sugaya, S. Kinoshita, and T. Chikama, “Novel configuration forlow-noise and wide-dynamic-range Er-doped fiber amplifier for WDMsystems,” in Optical Amplifiers and their Applications, 1995 OSATechnical Digest Series, Vol. (Optical Society of America, Washington,D.C.) 158-161).

It is an object of the present invention to provide a novel techniquewhich enables one to change an overall scale factor of a gain spectrumof an optical amplifier such as an EDFA without substantially changingthe shape of the gain spectrum. More generally, it is an object of theinvention to provide a technique for decoupling the magnitude and shapeof the gain spectrum of an EDFA or other optical amplifier. (The two arecompletely coupled in a conventional EDFA.) Thus, using the presentinvention, it is possible to provide an EFDA in a span which is shorterthan the design span without adding attenuation, by controlling thescale factor of the gain spectrum.

Gain can be obtained in an EDFA when some fraction of the erbium dopantions are excited into the metastable ⁴I_(13/2) state. A signal bandphoton (wavelength typically 1525-1600 nm) incident upon an excited ioncan stimulate the release of a photon identical to itself and cause theerbium ion to return the ⁴I_(15/2) ground state. Silica based EDFAs usedin telecommunication systems are typically pumped at wavelengths near(+/−˜25 nm) 1480 nm or 980 nm. As shown in FIG. 1, the former directlyexcites the ions from the ground state to the metastable level, whereasthe latter makes use of the auxiliary ⁴I_(11/2) state. Other pump bandsare possible, but lower power conversion efficiency is typically foundin 650 nm and 800 nm bands due to excited state absorption (ESA) andconsequently they are not used in commercial systems.

The gain spectrum of an EDFA can be approximately written as

G _(dB)(λ)=10 log₁₀(e)Γ(λ)L _(EDF) [{overscore (N₂+L )}σ_(e)(λ)−{overscore (N₁+L )}σ _(a)(λ)]  (1)

where G_(dB) is the amplifier gain in decibels, L_(EDF) is the totallength of the erbium-doped fiber (EDF) within the amplifier, σ_(e) andσ_(a) are the emission and absorption cross sections, respectively, andN₁ and N₂ are average populations (ions per unit volume) of the groundstate and metastable levels, respectively (C. R. Giles and E. Desurvire,“Modeling erbium-doped fiber amplifiers,” Journal of LightwaveTechnology, vol. 9, pp. 271-283, 1991). The local fractional averageinversion (i.e., inversion averaged over a cross section of the fiber ata particular axial point) is calculated as $\begin{matrix}{\overset{\_}{n_{i}} = {\frac{1}{L}{\int_{0}^{L}{{z^{\prime}}{\int_{0}^{{doping}\quad {radius}}{r{r}\quad {{N_{i}\left( {r,z^{\prime}} \right)}/\quad \left\lbrack {{N_{1}\left( {r,z^{\prime}} \right)} + {N_{2}\left( {r,z^{\prime}} \right)}} \right\rbrack}\quad {\left\{ {{i = 1},2} \right\}.}}}}}}} & (2)\end{matrix}$

which will be convenient for re-writing the form of Eqn. (1) below.

EDFAs are typically used such that nearly all erbium ions are in themetastable ⁴I_(13/2) level (level 2) or the ground state ⁴I_(15/2). Thisis because efficiency reducing ESA is likely to be a problem if this isnot the case. When pumping in the 980 band, ions are predominantly movedfrom the ground state to the ⁴I_(11/2) level. In silica this state has ashort lifetime on the order of 10 μs and the ions undergo non-radiativedecay to the metastable state. Because the lifetime of the ⁴I_(11/2)level is so much shorter than that of the metastable (at the powerlevels typically encountered in commercial communication systems) thepopulation of this level is typically negligible. In low phonon energyglasses (i.e., glasses with a phonon energy significantly lower than insilica) such as ZBLAN, the ⁴I_(11/2) level has a lifetime on the orderof 10 ms which is a large fraction of that of the metastable.Furthermore, there is an ESA within the 980 nm pump band which resultsin the excitation of an ion from the ⁴I_(11/2) to the ⁴I_(7/2) state andthis process can reduce the efficiency of the amplifier. As a result,EDFAs made out of low phonon energy glasses have typically not beenpumped in the 980 nm band. Recently, there has been work at finding 980nm band wavelengths which would provide efficient amplificationprimarily for obtaining a high inversion to get a low noise figure (goodnoise performance) (M. Yamada, Y. Ohishi, T. Kanamori, H. Ono, S. Sudoand M. Shimizu, “Low-noise and gain-flattened fluoride-based Er³+-dopedfiber amplifier pumped by 0.97 μm laser diode,” Optics Letters, vol. 33,pp. 809-810, 1997; M. Yamada, Y. Ohishi, T. Kanamori, S. Sudo and M.Shimizu, “Low-noise and gain-flattened fluoride-based Er³+-doped fiberamplifier pumped by 0.97 μm laser diode,” Optics Letters, vol. 22, pp.1235-1237, 1997). Good power conversion efficiencies have yet to bedemonstrated, though, so cost effective 980 nm band pumping has yet tobe proven. Since N₁+N₂ is approximately equal to the total number ofactive erbium ions in the amplifier, Eqn. 1 can be written in terms offractional populations as

G _(dB)(λ)=10 log₁₀(e)ΓN _(tot) L _(EDF) [{overscore (n₂+L )}(σ_(e)+σ_(a))−σ_(e)]  (3)

where n_(i)=N_(i)/N_(tot). In Eqn. 1, all of the variables on the rightside of the equation, except n₂, are fixed once the EDF has beenmanufactured, cut to length and constructed into an amplifier.Therefore, if the gain of the amplifier is to be changed, n₂, theaverage inversion, needs to be changed. However, ΔG_(dB)/Δn₂, the changein the gain per Δn₂ change in inversion, is proportional toΓ(σ_(e)+σ_(a)) which is spectrally dependent. As a result, the gain of aconventional EDFA cannot be increased or decreased in a spectrallyuniform manner. Instead, if steps are taken to increase or decrease thegain at a particular wavelength, the shape of the gain spectrum as afunction of wavelength will be distorted.

In view of the foregoing, it is a further object of the invention toprovide an EFDA in which the gain can be increased or decreased in aspectrally uniform manner. In other words, it is an object of theinvention to provide an EFDA or other optical fiber amplifier an overallscale factor of the amplifier gain spectrum may be adjustedsubstantially independently of the shape of the gain spectrum.

Dual or multiple wavelength (or hybrid) pumping of erbium-doped fiberamplifiers (EDFAs) has been proposed to achieve a number of goals (seeU.S. Pat. No. 5,710,659). For example, 980 nm pumping is typically usedin the first stage because it can achieve a full inversion of the erbiumions and thus attain the best possible noise performance. On the otherhand, 1480 nm pumped gain stages can have a better power conversionefficiency than 980 nm pumped stages (a smaller fraction of the lessenergetic 1480 nm photons are dissipated within the amplifier in orderto generate 1530 nm signal photons) and 1480 nm pump lasers may costless in some cases. Thus, a two stage amplifier in which the first ispumped with 980 nm and the second is pumped with 1480 nm may combinesome of these advantages. This approach could then be extended to asingle gain stage by simply pumping it from opposite ends with differentpump wavelengths. This provides the added benefit of making it possibleto combine multiple pumps.

In contrast to the foregoing, it is a particular objective of theinvention to provide an EDFA in which the gain can be increased ordecreased in a spectrally uniform manner using multiple pumpingwavelengths.

SUMMARY OF THE INVENTION

In the present invention, one or more auxiliary “pump” or controlwavelengths are used such that some fraction of the dopant ions may beintentionally placed into states other than the ground state or themetastable level such that N_(tot)>N₁+N₂. In this case, Eqn. (1) can bewritten as $\begin{matrix}{{G_{dB}(\lambda)} = {10\quad {\log_{10}(e)}\Gamma \quad L_{EDF}{N_{tot}\left\lbrack \frac{\overset{\_}{N_{1}} + \overset{\_}{N_{2}}}{N_{tot}} \right\rbrack}\left\{ {{\overset{\_}{n_{2}}\left\lbrack {{\sigma_{e}(\lambda)} + {\sigma_{a}(\lambda)}} \right\rbrack} - {\sigma_{a}(\lambda)}} \right\}}} & (4)\end{matrix}$

The shape of the gain spectrum in this case is again determined by therelative average inversion n₂, but the overall scale factor of the gainspectrum is now proportional to the term f_(act)=[(N₁+N₂)/N_(tot)] whichis typically unity or not controlled in current EDFAs. However,according to the invention, fat can be brought to less than unity andcontrolled such that the same gain spectral shape can be preserved atdifferent absolute gain values. In a more general sense, the shape ofthe amplifier's gain spectrum can be separated from the absolute gainitself. As a result, an amplifier can be designed to be a flexible“dynamic gain tilt” compensator. Its gain shape can be tuned to cancelimbalances arising from dynamic gain tilt in other amplifiers in acascade. Its absolute gain level could then be adjusted to match thepower levels needed by, e.g., link terminal equipment.

One illustrative embodiment of the invention, which implements theforegoing may be described as follows. An optical amplifier comprises anoptical fiber segment doped with impurity ions for providing opticalgain for an optical signal propagating in the optical fiber segment.

A first source of a first pumping wavelength pumps the ions from a firstground state to a second metastable state. The metastable decays stateto the ground by stimulated emission to provide the optical gain. Asecond source of a second pumping wavelength pumps the ions from theground state to a third auxiliary state. The auxiliary state decaysspontaneously down to the metastable state. Thus, by controlling thepumping power in one or both pumping wavelengths, it is possible tocontrol the fraction of ions in the metastable state. This in turnpermits control of an overall scale factor of the gain spectrum withoutsubstantially affecting the shape of the gain spectrum.

Illustratively,

(a) the ions are erbium

(b) the first pumping wavelength is 1480 nm

(c) the metastable state is ⁴I_(13/2)

(d) the second pumping wavelength is 980 nm, and

(e) the third state is ⁴I_(11/2).

Preferably, the optical fiber segment is formed of a low phonon energyglass such as ZBLAN. (Other low phonon energy glasses include telluritesand cesium aluminates). In this case, the lifetime of the auxiliarystate is a significant fraction of the lifetime of the metastable state.This makes it possible to control the population of the metastable stateand thus the scale factor of the gain spectrum by pumping a fraction ofthe ions into the auxiliary state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an abbreviated energy level diagram of Er3+.

FIG. 2 schematically illustrates an optical fiber amplifier according toan illustrative embodiment of the invention.

FIG. 3A and FIG. 3B are plots of pairs of gain spectra from the EDFA ofFIG. 2.

FIG. 4 is a plot of normalized gain difference derived from the data inFIG. 3A and FIG. 3B.

FIG. 5 is a plot which shows that an expanded range of gain spectra areavailable using the techniques of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with an illustrative embodiment of the invention, in anEDFA, one way to reduce f_(act), the fraction of the erbium ions thatare able to provide gain, is to pump an EDF that has a long lifetime(e.g. >1 ms) in the ⁴I_(11/2) level at both 1480 nm and 980 nm. Thetotal amount of power provided by each of the pump sources may beadjustable to generate the full range of available gain spectra. The1480 nm band pump directly moves ions from the ground state to the⁴I_(13/2) metastable level. The 980 nm band pump moves ions directlyfrom the ground state to the ⁴I_(11/2) auxiliary state. Erbium ionsexcited by the 980 nm band pump light on average are delayed by a timeequal to the lifetime of ⁴I_(11/12) level before spontaneously decayingdown to the metastable level where they can take part in theamplification process. As a result, some fraction of the erbium dopantions will occupy the ⁴I_(11/2) (and due to ESA higher levels) andf_(act) will be reduced below unity. The longer the lifetime of theauxiliary state, the less power it takes to “sideline” some fraction ofthe ions using the auxiliary or control pump laser.

In short, in accordance with the present invention, an optical amplifiersuch as an EDFA is operated so that a controllable fraction of theactive dopant ions are in an energy state such that they (temporarily)do not interact with the optical signal being amplified for the purposeof independently controlling the shape and magnitude of the gains of theoptical amplifier. Preferably, the time it takes for a dopant ion toleave the ground state and arrive at the metastable state via theauxiliary state takes at least 1 or 2 ms (in the case of ZBLAN, it is 10ms).

FIG. 2 schematically illustrates an optical fiber amplifier whichoperates in accordance with the present invention. The optical fiberamplifier 10 comprises an erbium doped optical fiber segment 20.Illustratively, the fiber segment 20 is a 4.6 m segment of ZBLAN dopedwith erbium at 1000 ppm. Preferably, the fiber 20 is formed from a lowphonon energy glass such as ZBLAN so that the auxiliary state ⁴I_(11/2)has a relatively long life. An optical signal to be amplified enters theamplifier 10 at the input port 22 and passes through the isolator 24.The optical signal has a wavelength in the 1525-1600 nm band or perhapsout to 1700 nm. The amplified optical signal passes through the isolator34 and exits the amplifier system via the output port 32. The amplifier10 includes two pumps 50 and 60. The pump 50 is a laser in the 980 nmband. The optical energy in the 980 nm pumping band is coupled into thefiber 20 by the WDM (Wavelength Division Multiplexer) 42 and propagatesin the fiber 20 codirectionally with the optical signal. The pump 60 isa laser in the 1480 nm band. The optical energy in the 1480 nm pumpingband is coupled into the fiber 20 by the WDM 44 and propagatescounter-directionally with the optical signal.

The invention can also be implemented with the reverse arrangement (980nm pump power counter-propagating with the signal and the 1480 nm bandpump co-propagating). Having the 980 nm pump power counter-propagatingwith the signal can be advantageous for reducing the noise figure of theamplifier if a pump wavelength near 970 nm is used (M. Yamada, Y.Ohishi, T. Kanamori, H. Ono, S. Sudo and M. Shimizu, “Low-noise andgain-flattened fluoride-based Er³+-doped fiber amplifier pumped by 0.97μm laser diode,” Optics Letters, vol. 33, pp. 809-810, 1997; M. Yamada,Y. Ohishi, T. Kanamori, S. Sudo and M. Shimizu, “Low-noise andgain-flattened fluoride-based Er³+-doped fiber amplifier pumped by 0.97μm laser diode,” Optics Letters, vol. 22, pp. 1235-1237, 1997). However,efficiency loss due to ESA will likely be higher in this configurationthan if the pump directions are reversed. Therefore, the optimumarrangement of the pumps will have to be determined on a case by casebasis.

FIG. 3A and FIG. 3B show gain spectra of the amplifier described aboveas measured with a broadband source and the time-domain extinctiontechnique. All spectra were measured on the same amplifier. The highergain spectra were measured using predominantly 1480 nm band pump light.The lower gain spectra were obtained by increasing the amount of pumplight at 980 nm and reducing the amount of 1480 nm pump light. The pairsof curves in FIG. 3A and FIG. 3B demonstrate how the gain wassignificantly reduced without the large change in gain shape thatnormally accompanies such a gain change.

This is further illustrated in FIG. 4 where the differences between thegain curves of FIGS. 3A and 3B are plotted with the dynamic gain tiltfunction measured for this amplifier using a single 1480 nm band pump.In all cases, the curves have been normalized by their peak value toindicate the wavelength dependence of the gain changes. The dynamic gaintilt function represents the amount of gain variation that one gets frompump/signal power adjustments alone which simply change the averageinversion. The relative variation in the data derived from FIG. 3A andFIG. 3B is significantly smaller than for a conventional amplifier. Theslope of the normalized gain difference (larger gain minus smaller)curves for the present amplifier can be made to have either a positiveor negative slope, or by proper interpolation, a zero slope. The latteris the typical arrangement, but the ability to change sign can be usedto provide compensation of gain slope errors in systems consisting ofmultiple amplifier technologies. To determine the appropriate amounts ofpump and auxiliary “control pump” light to apply for a specific need(such as providing the spectrally flattest gain reduction possible) onecan perform a series of measurements of the amplifier gain spectrumunder conditions which match the actual deployment conditions whileusing measured amounts of the various pump powers. A look-up tablerelating drive currents to gain shapes and input powers can then bederived. Intermediate gain spectra can then be interpolated between theexcitation powers used in the calibration measurements.

Because of the ability to independently control the magnitude of thegain from the amplifier and the effective average inversion of theamplifier (which determines the shape of the gain spectrum), one hasmuch more control of the spectrum of an amplifier than was previouslypossible. While Eqn. (3) predicts that the gain spectrum of a particularamplifier is constrained to a single parameter family of curves which donot cross, FIG. 5 shows that a significantly expanded range of gainspectra are available using the methods described here. For example, thetwo upper curves can be made to cross thereby providing higher gains tothe two ends of the gain spectrum.

Changes in pump wavelength within the 980 nm pump band of erbium-dopedsilica based fibers manifest themselves as changes in gain slope in thelonger wavelength portion of the erbium gain band (˜1540-1565 nm) (K. W.Bennett, F. Davis, P. A. Jakobson, N. Jolley, R. Keys, M. A. Newhouse,S. Sheih, and M. J. Yadlowsky, “980 nm band pump wavelength tuning ofthe gain spectrum of EDFAs,” in Optical Amplifiers and theirApplications, 1997 OSA Technical Digest Series, Vol. (Optical Society ofAmerica, Washington, D.C.) PD⁴-1-PDR4). Since the present invention canbe used to change gain slope in that region it provides a potentialmeans of compensating for this effect, though the powers needed may behigh in Er/aluminum doped silica.

The gain spectral control described above can be achieved through theuse of other combinations of pump wavelengths. For example, the ESA oferbium at 850 nm can be used to excite ions into the ⁴S_(3/2) level.From this state the typical decay path is a non-radiative cascade backto the metastable level. The time it takes for this process to occur,which is highly host material dependent, represents time during which anion cannot take pab in the 1.5 μm amplification process. The sum of allion s taking part in the ESA process, therefore, reduces f_(act). Theuse of ESA around 850 nm to separately control f_(act) and the averageinversion may provide more independent control of these parameters. Byoptimizing the wavelength around the ESA peak to maximize the ratio ofESA to ground state absorption (GSA), the pump function and the gaincontrol operation can be decoupled relative to the 1480 nm 980 nm bandpumping, though this may in some cases come at the expense of requiringhigher total pump powers. Similar arguments hold for other ESA lines oferbium (e.g., 1140 nm, 790 nm etc.) (or other dopants). The 850 nm ESAband, however, has the practical advantage that low-cost GaAs diodelaser technology is available in the appropriate wavelength band. Thepower required at the auxiliary “pump” or ESA wavelength will, ingeneral, be related to the time is takes for the ion to spontaneouslydecay from the “trapping” state or states back into the ground state orthe metastable level and again participate in the amplification process.The longer this process takes, the less power that will be needed, allother things being equal. This suggests the use of ESA levels with longlifetimes, or levels which decay into long lifetime levels. Furthermore,it suggests the use of low phonon energy glasses for which the excitedstates will generally have longer lifetimes. An exception to the aboveargument occurs when an additional ESA process happens which hasdetrimental effects. Lacking pathological outcomes, additional ESA maynot pose a problem. The potential with higher order ESA includesexcitation of states with shorter lifetimes than the target (therebyrequiring additional auxiliary “pump” power) or upconversion cascadeswhich promote an ion over the band-gap and potentially lead tophotodarkening.

The above approaches should also be applicable to rare-earth dopedamplifiers other than erbium if the dopants have the appropriateauxiliary energy levels outside of the amplifying transition.

Finally, the above described embodiments of the invention are intendedto be illustrative only. Numerous alternative embodiments may be devisedby those skilled in the art without departing from the spirit and scopeof the following claims.

What is claimed is:
 1. A method for operating an optical amplifiercomprising an optical fiber segment doped with impurity ions comprisingthe steps of: (1) pumping said optical fiber segment with a firstpumping wavelength, (2) pumping said optical fiber segment with a secondpumping wavelength, (3) selecting a pumping power of at least one ofsaid first and second wavelengths to thereby select a scale factor for again spectrum of said optical amplifier without changing a shape of saidgain spectrum as a function of wavelength.
 2. The method of claim 1wherein said ions are erbium ions.
 3. The method of claim 1 wherein saidoptical fiber segment is formed from a low phonon energy glass.
 4. Themethod of claim 1 wherein said glass is a tellurite or a fluoride glass.5. The method of claim 4 wherein said optical fiber segment is formedfrom ZBLAN glass.
 6. The method of claim 1 wherein one of said first andsecond pumping wavelengths is 980 nm and the other of said first andsecond pumping wavelengths is 1480 nm.
 7. The method of claim 1 whereinsaid selecting step comprises the step of selecting a pumping power ofboth said first and second pumping wavelengths.
 8. The method of claim 1wherein one of said pumping wavelengths propagates codirectionally withan optical signal propagating in said optical fiber segment and theother of said pumping wavelengths propagates counter directionally withsaid optical signal in said optical fiber segment.
 9. A method ofoperating an optical amplifier comprising an optical fiber segment dopedwith impurity ions comprising the steps of: (a) pumping said ions with afirst pumping wavelength from a first ground state to a secondmetastable state, said metastable state decaying to said ground statevia stimulated emission to supply optical gain to an optical signalpropagating in said optical fiber segment, (b) pumping said ions with asecond pumping wavelength from said ground state to a third state, saidthird state decaying to said metastable state, and (c) selecting a scalefactor for an optical gain spectrum of said optical amplifier withoutsubstantially varying a shape of said optical gain spectrum as afunction of wavelength by controlling a pumping power of at least one ofsaid first and second pumping wavelengths and thereby controlling afraction of said ions in said metastable state.
 10. The method of claim9 wherein said optical fiber segment comprises a low phonon energyglass.
 11. The method of claim 9 wherein (a) said ions are erbium ions,(b) said first pumping wavelength is 1480 nm (c) said metastable stateis ⁴I_(13/2) (d) said second pumping wavelength is 980 nm, and (e) saidthird state ⁴I_(11/2).
 12. The method of claim 11 wherein said selectingstep comprises selecting a pumping power of said 980 nm pumpingwavelength.
 13. The method of claim 9 wherein one of said pumpingwavelengths propagates codirectionally with said optical signal in saidoptical fiber segment and the other of said pumping wavelengthspropagates counter-directionally to said optical signal in said opticalfiber segment.
 14. An optical fiber amplifier comprising: an opticalfiber segment doped with impurity ions for providing optical gain for anoptical signal propagating in said optical fiber segment; a first sourceof a first pumping wavelength for pumping said ions from a first groundstate to a second metastable state, which metastable state decays tosaid first ground state via stimulated emission to supply said opticalgain; a second source of a second pumping wavelength for pumping saidions from said ground state to a third state, which third state decaysto said second metastable state; and a circuit for selecting a scalefactor for an optical gain spectrum of said optical amplifier withoutvarying a shape of said optical gain spectrum as a function ofwavelength by controlling a pumping power of at least one of saidpumping wavelengths, and thereby controlling a fraction of said ions insaid metastable state.
 15. The optical fiber amplifier of claim 14wherein said optical fiber segment comprises a low phonon energy glass.16. The fiber amplifier of claim 15, wherein: (a) said ions are erbiumions; (b) said first pumping wavelength is 1480 nm; (c) said metastablestate is ⁴I_(13/2); (d) said second pumping wavelength is 980 nm; and(e) said third state is ⁴I_(11/2).
 17. An optical fiber amplifiercomprising: an optical fiber segment formed from a low phonon energyglass and doped with impurity ions for providing optical gain for anoptical signal propagating in said optical fiber segment, a first sourceof a first pumping wavelength for pumping some of said ions from a firstground state to a second metastable state, which metastable state decaysto said first ground state, and a second source of a second pumpwavelength for pumping some of said ions from said ground state to athird auxiliary state, which auxiliary state decays to said secondmetastable state.
 18. The amplifier of claim 17, wherein the time ittakes an ion to go from said ground state to said metastable state viasaid auxiliary state is at least 1 ms.
 19. A method for operating anoptical amplifier comprising an optical fiber segment doped withimpurity ions to amplify an optical signal propagating in said fibersegment, said method comprising the steps of: pumping said optical fibersegment with a first pumping wavelength and a second pumping wavelength,said that a controllable fraction of the ions are in an energy statesuch that they do not interact with said signal propagating in saidfiber segment so as to independently control a shape and magnitude of again spectrum of said amplifier.